Through custom and convenience, the preferred connector for general purpose use on many items of test equipment is BNC female (BNC stands for Bayonet Navy Connector). The BNC female connector has a female shell, or cylindrical shield, whose outer surface carries two opposing bayonet pins that engage respective spiral grooves and detents in a bayonet latch that is part of the male BNC connector. The actual RF connection is made between male and female center conductor portions and between male and female cylindrical shield portions. To connect the center conductors, a male pin has a reduced diameter portion that extends beyond a shoulder. The male pin enters a female socket whose outer diameter matches that at the shoulder of the male pin. In this way the mated male and female center conductor portions exhibit no change in outer diameter, provided that they are indeed fully mated. In a similar manner the cylindrical shield around the male pin has an outer diameter that just fits inside the larger cylindrical shield over the female pin. The larger (female) cylindrical shield has an interior step to a reduced diameter that matches the inside diameter of the smaller (male) cylindrical shield over the male pin. When the center conductors are fully mated the smaller cylindrical shield will enter and exactly bottom out against the step in the larger cylindrical shield, and any change in shield inside diameter will vanish, with the result that both the center conductor and the surrounding cylindrical shield each appear to have constant diameters. This mechanical arrangement of overlapping penetration is such that the center conductor and shield are held in rigid coaxial alignment, despite the presence of a mechanical joint. In an ordinary BNC connector, a spring in the back of the BNC latch provides a modest amount of force to cause the full mating of the center pin and the shields. One end of this force is anchored by the detent of the bayonet latch engaging the bayonet pins, and allows the mated parts to be forced together. (This rather abbreviated discussion of the BNC connector technique does not address all issues associated with the BNC design over it long history, such as the use of Teflon, axial slits in the male cylindrical shield, and cable attachment methods. But it is sufficient to raise the issues we are interested in.)
A disadvantage to the original BNC design is that the spring can weaken with age and severe use, and that anything, such as the weight of a long cable or of a probe pod or other housing at the male end, that pulls the male connector away from the panel by overcoming the spring will also cause the mated center conductors and mated shields to each separate to a greater or lesser degree. The resulting diameter variations introduce abrupt changes in characteristic impedance, causing undesirable reflections for signals at high frequencies.
U.S. Pat. No. 6,609,925 issued 26 Aug. 2003 and entitled Precision BNC Connector discloses an arrangement wherein the aforementioned spring is replaced by a deliberate (non-resilient) displacement produced by the rotation of a knurled outer shell engaged by threads to the BNC latch. When the knurled outer shell is turned in the proper direction after an initially mating of the connector, the male pin and its surrounding cylindrical shell are driven forward to fully mate with their female counterparts. As before, the bayonet pins serve as an anchor for the force involved.
Now, it is not that the arrangement described in U.S. Pat. No. 6,609,925 does not work: it does. But there are situations where an aspect of its operation is inconvenient. That is, it is at odds with a human usage model arising out of expectations formed by using other connectors. We shall describe one such situation in order to illuminate a desired property of the improved connector to be described in due course.
Suppose that the instrument or item of test equipment has an input channel using a panel mounted female BNC connector. It is conventional that such connectors are quite rigidly attached to the panel, and do not translate, pivot or rotate once installed. Now let a similar connector be on the panel, some distance away. The second connector is the source of a calibration signal that the user of the instrument wishes, from time to time, to apply to the input channel. The manufacturer of the instrument supplies a high quality (and expensive!) xe2x80x9ccalibration cablexe2x80x9d that is to be used to make the interconnection. The calibration cable might be a length of rigid xe2x80x9chard linexe2x80x9d coaxial cable, or semi-rigid cable. Or, it might be flexible, in that it can be bent somewhat, but will resist (and not undergo without damage)torsional rotation, or twisting. (There are other, non-calibration situations where test equipment sometimes has an externally made connection, such as applying by a short coaxial cable either an internally generated or externally supplied standard frequency, or other signal, to an input that uses it. It will be appreciated that these other situations are also represented by the xe2x80x9ccalibrationxe2x80x9d example we are about to pursue.)
Suppose, as point of departure, that such a calibration cable was in the shape of a shallow broad U and had conventional BNC male connectors at each end. It is typically fairly short, say, six to twelve inches. It might bend, but not with a small radius, and the two 90xc2x0 bends of the U shape mean that the length of the cable is already fairly well consumed just to give it that shape. To attach it, one would likely grasp, between thumb and forefinger, each male BNC latch with different hands and align the cable mounted connectors with their panel mounted counterparts. Because the bayonet pins are located some distance back from the end of the panel mounted female connector, some coaxial engagement is possible before further engagement along the axis requires that the bayonet pins actually enter the grooves in the BNC latch on the male connector. Engagement of the bayonet pins with the grooves requires rotational alignment. The BNC latch is typically allowed to rotate freely, so that such alignment is possible. Typically, the operator rotates the BNC latch with wrist motion or by rolling it between the thumb and forefinger. Once the bayonet pins enter the grooves of the latch, a forward motion and further twisting of the latches will connect the calibration cable. The xe2x80x9conlyxe2x80x9d problem here is the low quality of the connection formed by a conventional non-precision BNC connector. Unfortunately, for a calibration signal in the gigahertz region, unsatisfactory connectors can make it appear that the instrument does not meet its specifications. It is for that and other reasons that there are such things as precision BNC connectors.
Now, let""s repeat the same operation with the precision BNC connector of U.S. Pat. No. 6,609,925. Suppose, for the moment, that the connectors are cable borne BNC male connectors (that is, they are directly attached to the calibration cable instead of being cross series adapters as shown in the patent). It won""t work unless the cable can be twisted as it leaves the connector, or, unless after the 90xc2x0 bend that is half of the U-shaped bend, the cable can be further bent to make the U into a W, and then (later on) un-bent back into a U again. This is because the back side of that connector (the part that attaches to the cable or that carries the xe2x80x9cadapter partxe2x80x9d) cannot be rotated relative to the BNC latch. (For those that care to look at FIG. 3 of U.S. Pat. No. 6,609,925, it is because dogs 40 can only slide, and not rotate, in slots 41 of male shell 39, and because element 50xe2x80x94representing the cable or adapterxe2x80x94screws tightly into shell 39.) So, in order to engage the bayonet pins of, say, the left-hand pair of connectors, one would have to rotate the left-hand BNC shell about 90xc2x0 clockwise (as seen from behind) in order to get the spiral portion of the grooves to traverse over the bayonet pins until those bayonet pins enter the detents. That means that a cable assembly that does not permit twisting has to also rotate as the latch is twisted. But then how can the right-hand connectors then stay engaged with each other? (One could pull the right-hand connectors apart (by distorting the U-shape) to allow the whole cable to rotate about the left-hand connectors, but in the end it will not help. Read on.) For the right-hand connectors to remain engaged (even if not yet fully mated) would require that the calibration cable be compliant, either by twisting as it enters a male connector being rotated (which we assume that it will not do), or that it be extra long in the middle of the U, so that it can bend in a couple of places to temporarily become a W. How gross! And what an undignified thing to do to an expensive length of high quality cable. (Not to mention, suppose it is hard line . . . ) Now, with the left-hand connectors mated, the same difficulties are repeated to mate the right-hand connectors, save that it now a little worse, since the left-hand connectors are now rigidly held in place. The upshot is, the calibration cable cannot be easily installed if it is merely bendable and cannot be twisted, and almost cannot be installed at all if it is rigid. Removal of the cable presents the same problems in reverse.
Those familiar with high quality coaxial cable (e.g., Sucoflex microwave cable from Huber+Suhner) will appreciate that, besides being rather expensive, such cable cannot be twisted, is stiff, and does not bend abruptly. These various properties of the cable mean that we have not inflated the calibration cable example to make it appear worse than it really is.
A related example exists when the connectors on the panel are precision BNC connectors, and the calibration cable is equipped with SMA or, better still, APC 3.5 connectors. Now we have a really high quality (and more expensive still) calibration cable to deal with, which perhaps has other uses as well. We continue to assume that it does not twist, and that abrupt bending of it is discouraged. Now let the precision BNC female connectors on the panel each receive a precision cross series adapter to match the style of connector used by the calibration cable. As before, the nature of the cross series adapter is as shown in U.S. Pat. No. 6,609,925, and the xe2x80x9cadapter partxe2x80x9d does not rotate relative to the BNC latch part. Now, one could proceed simply by mating and tightening the nut-like shells of the connectors on the calibration cable, just as though the instrument originally had those different style connectors instead of BNC. This works, but is, unfortunately, not a pleasant experience, either. The nut-like shells are small, hard to tighten correctly with thumb and forefinger (a special torque wrench is often used by metrology purists), and repeated use of the connectors exposes them to damage and degradation. After a few bouts of sore fingers, and to protect the expensive APC 3.5 connectors, our operator decides to leave the cross series adapters connected to the calibration cable, expecting that it will be much easier to rotate the larger BNC latches, and also expecting that the more mechanically robust and better protected precision BNC connectors will be a better choice for repeated mounting and un-mounting, anyway. The motives are sound, but the bad news is that we are back to the first example of where something has to twist or bend. Either the connectors on the ends of the calibration cable have to be loosened so they can twist in place (ugh! ) and then re-tightened (thus nullifying any advantage and probably inflicting unneeded rotational wear on the mating surfaces of those expensive connectors . . . ), or, the cable has to twist at the connector or bend additionally between the legs of the U (which it either won""t or shouldn""t).
So, how can we retain the ease of use and electrical performance advantages of the precision BNC connector described in U.S. Pat. No. 6,609,925 while using it (or something like it), either as a male connector mounted directly to the ends of the calibration cable, or, as part of precision cross series adapters that are left permanently attached to the ends of the calibration cable. It would seem that something has got to rotate. What to do?
A solution to the problem of precision locking BNC male connector installation requiring twisting of the cable or multiple bends to accommodate the rotation of the BNC latch is to arrange that the shell portion of the male connector that carries the adapter connector or cable clamp on one end and that is the male cylindrical shield at the other end, is free to rotate whenever the precision locking BNC male connector is not locked, whether or not it is mated with a female connector. A knurled sleeve, or draw nut, is captive at a location along the male shell, but is free to rotate. The knurled sleeve has internal threads that engage external threads on a portion of the BNC latch. A radial friction device is in contact with both an external surface of the BNC latch and the internal surface of the knurled sleeve, at a location adjacent to the aforementioned external and internal threads. When not engaged with the bayonet pins of a female connector, rotating the knurled sleeve will rotate the BNC latch also, by virtue of the friction device, but both will, as a unit, rotate freely relative to the shell. Once the bayonet pins engage the spiral portion of the slot in the BNC latch, the friction between the sleeve and the latch is sufficient to rotate the latch (CW as viewed from the rear) all the way into the detent. At that point the latch can turn no more, and further CW rotation of the sleeve by about three-quarters of a turn causes thread driven displacement of the male shell toward the female parts by about 0.030 inches. This applies the compression that produces the locked condition. To unlock the connectors the knurled sleeve is turned in the CCW direction. The friction device does not transmit enough torque to overcome the detent, so that the shell initially stays still as the knurled sleeve rotates about it, which undoes the thread induced displacement until no more displacement in the other direction is possible. A spring washer assists in keeping the bayonet pins and the detents engaged until the draw nut has been rotated enough to provide sufficient linear clearance for their non-binding release. When no further displacement is available the knurled shell will not rotate further in the CCW direction without transmitting that rotation to the latch. After the CW three-quarters of a turn is undone by CCW rotation, the male and female shells are no longer urged together, and further CCW rotation of the knurled sleeve is, through the lack of further thread travel, transmitted to the BNC latch, which causes it to leave its detent and traverse the spiral over the bayonet pins to where they are opposite the entrance to the groove, whereupon a simple axial tug separates the connectors. The friction device may be a neoprene washer held between two adjacent metallic washers.